1. Field of the Invention
This invention relates to novel inhibitors of androgen synthesis that are useful in the treatment of prostate cancer and benign prostatic hypertrophy. The present invention also provides methods of synthesizing these novel compounds, pharmaceutical compositions containing these novel compounds, and methods of treating prostate cancer and benign prostatic hypertrophy using the androgen synthesis inhibitors of the present invention.
2. Description of the Related Art
Prostate cancer now ranks as the most prevalent cancer in men. Approximately 160,000 new cases are diagnosed each year; of these, 35,000 will die of metastatic disease. In women, breast cancer kills 45,000 women per year. The present inventors have previously proposed that selective aromatase (estrogen synthetase) inhibitors to control estrogen production would be potentially useful agents for breast cancer. In men, aromatase inhibitors may be useful for conditions associated with estrogen excess, such as gynecomastia and oligospermia (Coen et al., 1991; Hsiang et al., 1987). It has been suggested that aromatase inhibitors might also be of value in prostatic cancer and benign prostatic hypertrophy (BPH) (Henderson et al., 1991).
In 1973, the present inventors reported the first of a number of compounds which are potent and selective inhibitors of aromatase (Schwarzel et al., 1973). The most active of those inhibitors, 4-hydroxyandrostene-3,17-dione (4-OHA) (Brodie et al., 1976), was found to act by rapid competitive inhibition followed by inactivation of the enzyme in vitro which appeared to be long-lasting or irreversible (Brodie et al. 1981). Enzyme inhibitors with these properties are thought to bind to the active site of the enzyme, are likely to be quite specific and should have long-lasting effects in vivo due to inactivation of the enzyme (Sjoerdsma, 1981). The present inventors further demonstrated that 4-OHA reduces peripheral plasma estrogen levels and causes significant regression of breast cancers in postmenopausal patients with advanced metastatic disease who have relapsed from other hormonal treatment, such as ovariectomy and tamoxifen. The compound has both oral and parenteral activity and is without significant side effects in these patients (Goss et al., 1986; Coombes et al. 1987). 4-OH-A (formastane) is now approved for treatment of breast cancer in many countries worldwide, including most European countries and Canada since 1995. It is the first new treatment for breast cancer in 10 years.
In men, estrogens are produced by the testes and by peripheral aromatization of adrenal androgens. Testosterone is the major product of the testis and is converted by 5.alpha.-reductase to the more potent androgen, dihydrotestosterone (DHT), in the prostate (Bruchovsky et al., 1968). While androgens are of primary importance in the growth of normal prostate, benign prostatic hypertrophy (BPH) and prostatic cancer, several lines of evidence suggest that estrogens also may have a role (Mawhinney et al, 1976).
4-OHA also inhibits 5.alpha.-reductase in vitro, although with less potency than it inhibits aromatase (Brodie et al. 1989b). Because of these two activities, the possibility that 4-OHA might be effective in prostatic cancer was explored in a small group of men with advanced disease. Subjective responses were observed in 80% of these patients, although there was no clear evidence of objective remissions (Shearer et al., 1991). Estrogen levels were reduced as expected but DHT concentrations were unchanged in the patients. The latter finding in addition to the weak androgenic activity of the compound may have determined the lack of objective responses.
Chemotherapy is usually not highly effective and is not a practical option for most patients with prostatic cancer because of the adverse side-effects which are particularly detrimental in older patients. However, the majority of patients initially respond to hormone ablative therapy although they eventually relapse, as is typical with all cancer treatments. Current treatment by orchidectomy or administration of gonadotropin-releasing hormone (GnRH) agonists result in reduced androgen production by the testis but does not interfere with androgen synthesis by the adrenals. Following 3 months of treatment with a GnRH agonist, testosterone and DHT concentrations in the prostate remained at 25% and 10%, respectively, of pretreatment levels (Foti et al., 1989). Similarly, about 20% of castrated patients in relapse had significant levels of DHT in their prostatic tissue (Geller et al., 1984). These findings suggest that the adrenals contribute precursor androgens to the prostate. This is supported by clinical studies of patients receiving combined treatment with either GnRH or orchidectomy and an antiandrogen, such as flutamide, to block the actions of androgens, including adrenal androgens. Such patients have increased progression-free survival time compared to patients treated with GnRH agonist or orchidectomy alone (Crawford et al., 1989; Labrie et al., 1993).
Although patients initially respond to endocrine therapy, they frequently relapse. It was reported recently that in 30% of recurring tumors of patients treated with endocrine therapy, high-level androgen receptor (AR) amplification was found (Visakorpi et al., 1995). Also, flutamide tended to interact with those mutant AR and stimulate prostatic cell growth. This suggests that AR amplification may facilitate tumor cell growth in low androgen concentrations. Thus, total androgen blockade as first line therapy may be more effective than conventional androgen deprivation by achieving maximum suppression of androgen concentrations which may also prevent AR amplification (Kellens, 1993). It is presently unclear whether sequential treatment with different agents can prolong the benefits of the initial therapy. New agents which act by different mechanisms could produce second responses in a portion of relapsed patients. Although the percentage of patients who respond to second-line hormonal therapy may be relatively low, a substantial number of patients may benefit because of the high incidence of prostatic cancer. Furthermore, there is the potential for developing more potent agents than current therapies, none of which are completely effective in blocking androgen effects.
The 17.alpha.-hydroxylase/C.sub.17,20 -lyase is a key enzyme in the biosynthesis of androgens and converts the C.sub.21 steroids (pregnenolone and progesterone) to the C.sub.19 androgens, dehydroepiandrosterone (DHEA), 5-androstenediol (A-diol), testosterone, and androstenedione in the testis and adrenals. Some inhibitors of 17.alpha.-hydroxylase/C.sub.17,20 -lyase have been described (Barrie et al., 1989; McCague et al., 1990; Jarman et al., 1990; Ayub et al., 1987; Nakajin et al., 1988, 1989; Angelastro et al., 1989; Potter et al., 1995). Ketoconazole, an active irnidazole fungicide, is the only inhibitor used currently to reduce testosterone biosynthesis in the treatment of patients with advanced prostatic cancer (Trachtenberg et al., 1984; Willimas et al., 1986). However, ketoconazole is not very potent. Moreover, it has a number of significant side-effects, including inhibition of several other cytochrome P.sub.450 steroidogenic enzymes, and reduction of cortisol production. Another drug used for prostate cancer, aminoglutethimide (AG), has similar drawbacks. This suggests that more potent and selective inhibitors of this enzyme could provide useful agents in treating this disease. In addition such compounds may be effective in treating breast cancer patients. AG was used for this purpose, but was associated with adverse side-effects.
In the prostate, 5.alpha.-reductase is the enzyme that converts testosterone to the more potent androgen, DHT, which stimulates prostatic growth. This enzyme occurs in two important isoforms, the Type I isoform expressed in human non-genital skin, and the Type II isoform present in the human prostate (see, Russell et al., 1994). The 5.alpha.-reductase inhibitor, N-[1,1-dimethyl-3-oxo-4-aza-5.alpha.androst-1-ene-17.beta.-carboxamide (finasteride; Merck) recently approved for treatment of BPH (Stoner, 1990) is a more potent inhibitor of the Type II than of the Type I isoform. However, finasteride is effective mainly in BPH patients with minimal disease, possibly because serum DHT levels have been found to be incompletely reduced (65-80%). As the Type I isoenzyme is probably the source of much of the residual plasma DHT, compounds that inhibit Type I as well as Type II may be more effective in patients. More recently, another azasteroid MK-434 has been described which reduces prostatic DHT levels in dogs more effectively than finasteride (Cohen et al., 1995). The main advantage of this compound, which has similar activity to finasteride in vitro, appears to be its more favorable pharmacokinetics. However, its efficacy in humans remains to be seen. Although these compounds reduce DHT levels, they also increase serum testosterone levels. Preservation of testosterone levels may be an advantage in patients with BPH. However, inhibitors of 5.alpha.-reductase which increase testosterone levels may not be sufficiently effective in treating prostatic cancer. While DHT binds to the androgen receptor with higher affinity than testosterone and dissociates more slowly, testosterone can bind to the receptor when DHT levels are reduced (Gormley, 1991). As indicated above, despite significant reductions in prostatic DHT levels during treatment (Cohen et al, 1995), these compounds are not as effective as castration. More importantly, it appears that they are less effective in eliciting prostatic cell death. The androgen-responsive gene, TRPM-2 associated with apoptosis is significantly enhanced by castration but not by finasteride treatment (Rittermaster et al., 1991; Shao et al, 1993). This has been attributed to the lower androgen levels after castration (Shao et al., 1993), which is mainly a consequence of the reduction in testosterone production. Recent studies of patients receiving long-term treatment with finasteride found some patients developed gynecomastia which led to breast cancer in a few cases (NEJM, September, 1996, letter to the editor). This raises concerns about the use of 5.alpha.-reductase inhibitors since blockade of this step increases the conversion of androgen substrates to estrogens. Compounds which reduce production of testosterone and DHT as well as other androgens by inhibiting 17-hydroxylase/lyase would not be associated with this problem and may be more effective in treatment of prostatic cancer.